The present invention relates to a radial force measuring cell capable of producing periodic output signals as a function of a periodic displacement of said force about said cell, said measuring cell comprising at least a set of transducers responding to the constraints produced by said force upon said cell, said transducers being arranged symetrically within two planes enclosing a determined angle between themselves.
It is necessary in many cases of the practice to be able to measure with a certain accuracy a force acting radially upon an axle. Such a need arises e.g. during the surveillance of technical installations, operating machines, hoisting appliances, etc. When it is necessary to determine e.g. the tractive force into a cable passing on a pulley it would be very convenient to utilize the axle of the pulley as a force measuring axle or cell.
In practice, the force to be measured may act at an angle constant or variable with respect to a determined direction which is generally choosen as the one of the greatest sensivity and designated by "direction of the load". It is therefore necessary that the measuring cell be not only capable of determining the force independently of its angle of incidence, but also the variable component of the force in the direction of the load.
The U.S. Pat. No. 3,695,096 describes a measuring cell of tubular configuration with a circular cross-section in the interior of which are arranged pairs of strain gauges in two perpendicular planes, these planes being tilted with respect to the axis of the cell at an angle of 45.degree.. The cell comprises at its outer surface, toward its extremities, regions of concentration of force having a reduced diameter and the strain gauges are mounted in the plane of these regions. The strain gauges respond to the constraints of shear. The cell is capable of measuring a force acting radially independently from the angle of incidence of this force, as well as the components of this force according to two directions of the load or directions of reference perpendicular between themselves.
From the U.S. Pat. No. 3,365,689 a resilient disc is known to be utilized for the same purpose as the preceding device and to comprise four radially extended drillings in which the transducers are disposed.
The published application DE-OS No. 25 18 548 describes also a measuring cell of tubular configuration with a circular cross-section in which the transducers (strain gauges) are disposed on plane surfaces provided on the outer mantle of the cell.
However, it is significant that none of the cells known from the prior art is available on the market as an independent measuring device. Tests effected with a measuring cell having a configuration similar to the one described in U.S. Pat. No. 3,695,096 have clearly shown that this cell is not utilizable either for the measurement of the component of the radial force with respect to a direction of the load, nor for the measurement of the same force independently of its angle of incidence. This is because strong errors occur in the measurement which are inherent in the principle itself of the utilization of the cell. The results of the measurements are summarized in FIGS. 1 and 2. FIG. 1 shows the components P cos .phi. (A) and P sin .phi. (B) of a force P acting radially on the measuring cell, these components being measured by two measuring bridges comprising transducers disposed within two perpendicular planes, the individual transducers being connected in the respective bridges in such a way as to eliminate the components of shear so that only the components of flexion are measured.
It is clearly visible that the components which are measured as a function of the angle of rotation .phi. present relatively important variations which are unacceptable in practice with respect to the theoretical sinusoidal desired values A' and B'. These differences may be explained by the fact that the measuring cell which rests freely at each of its extremities on a support is loaded in its middle by the radia force P to be measured. This arrangement gives rise to an irregular distribution of the forces in the various sections of the cell which produces a flattening of the top of the sinusoidal function. A Fourier analysis shows the presence of odd harmonies (3rd, 5th, etc).
FIG. 2 shows the curve of errors (errors in %) obtained during the measurement of the value of the force P. In principle, the value of the measured force P should be independent from the angle of rotation .phi.. However, it appears clearly that this is not the case; the errors of measurement being comprised between +21.5% and -6.5% as a function of the angle of rotation. FIG. 2 shows further that the curve of error has a period of 120.degree. corresponding to the presence of a third harmonic as mentioned above. This renders the cell entirely unsuitable for a practical utilization. The present invention seeks to avoid such errors of measurement in order to render the measurement of the value of the force P practically independent from the angle of rotation, that is with an error acceptable for the practical needs.
From the mentioned prior art and from the results of the measurements indicated above, it is the object of the present invention to realize a measuring cell of tubular configuration for reproductively measuring without any significant error (a) the component of a radially acting force with respect to a determined direction of reference as a function of the angle of rotation of said force and (b) the absolute value of the force independently of the angle of incidence of said force.